1. Field of the Invention
This invention pertains to a primer for synthetic plastic substrate parts to be joined with a cyano-acrylate adhesive and to the use of primer in an adhesive bonding process.
2. Statement of Related Art
It is known that molded articles made of synthetic plastics, especially nonpolar plastics such as polyethylene, polypropylene, polybutylene, polyoxymethylene, polytetrafluoroethylene, polyurethane, polysilicones, and the like can normally be adhesively bonded together only after the surfaces of the molded articles have been subjected to a special pretreatment; see Ullmann's Encyclopedia of Industrial Chemistry, Vol. A1, p. 254 (Verlag Chemie, Weinheim, Germany, 1985). Through the known pretreatments, the surface energy of the molded parts is increased, generally specifically by oxidation. Typical pretreatment methods include treatment with oxidizing flames, oxidizing chemicals or electrical discharges.
It is also known that the surfaces of nonpolar plastics to be bonded with cyanoacrylate adhesives may be pretreated with primers containing solutions of certain active ingredients in suitable organic solvents. Thus PCT-WO 88/10289 describes the use of various diazabicyclo or triazabicyclo compounds in solvents such as acetone, methyl ethyl ketone, chlorinated hydrocarbon(s), or methanol as a primer for the above-mentioned purpose. In this process, the primer solutions are applied to the surfaces to be bonded; following evaporation of the solvent, application of the cyanoacrylate adhesives and bonding of the surfaces thus treated takes place. Other primer materials include primary aliphatic amines (described in EP-A 0,295,013), quaternary ammonium compounds (described in EP-A 339,448), derivatives of lutidine, picoline, pyridine and 1-vinylimidazo {described in Chemical Abstracts 106:197637z, 197638a (1987)}, and various quinoline and pyridine derivatives (described in JP 62/195071).
However, a substantial drawback of some of the primer substances named is that they accelerate the curing of the cyanoacrylate adhesives without improved bonding to the substrate materials taking place. Frequently--because of spontaneous crosslinking--a distinctly lower adhesion results in comparison to non-pretreated surfaces. A further drawback of the primers known from PCT WO 88/10289 and EP-A 0,295,013 is the fact that the subsequent coating with the cyanoacrylate adhesives must take place as quickly as possible after application of the primer, since otherwise the strength of the bond diminishes. It should also be noted with respect to the vinylimidazole known as a primer that this compound is classified as extremely hazardous to health.
In Japanese Patent Application JP 02-45572 A, a primer for cyanoacrylates is described, the active substance of which is characterized by an imidazole ring. Specifically, the following are mentioned: imidazole, 2-methyl-imidazole, 2-ethyl-4-methyl-imidazole, 1-hydroxyethyl-2-methylimidazole, 2-amino-benzimidazole, 5-nitro-benzimidazole and N{(p-2-benzimidazole)phenyl} maleic acid imide. Higher shear strengths are achieved with these than with dimethylaniline, p-toluidine, or diethylamine primers. Nevertheless, the strengths achieved are not high enough for many applications.
Thus a major object of this invention is improving the handling and use properties of primers for cyanoacrylate adhesives, especially eliminating the above-mentioned drawbacks, thus prolonging the available intermediate storage or "open" time between primer application and application of the adhesive while maintaining the highest possible strength of the bond formed.